1. Field of the Invention
This invention relates to a circuit for connecting a pair of balanced signals selectively to separate loads or differentially to one of the loads while short circuiting the other load. In particular, it relates to a circuit for connecting balanced transformerless amplifiers selectively to corresponding loudspeakers or differentially to a single loadspeaker while short circuiting the unused loudspeaker.
2. The Prior Art
In 4-channel stereophonic sound reproduction four loudspeakers, each of which may include one or more drivers, if desired, are placed at the corners of a quadrilateral area. It is anticipated that the audience will face in the general direction of two of the speakers, which are referred to as the left-front and right-front speakers, and that the other two speakers will be behind the audience and are referred to as the left-back and right-back speakers. Four corresponding amplifiers are provided a left-front, a right-front, a leftback, and a right-back amplifier. If suitable audio signals from these amplifiers are presented to the four speakers, a high degree of realism may be obtained in the reproduced sound.
However, much information is recorded or transmitted only in the form of 2-channel stereophonic signals that do not include the proper information to reproduce 4-channel sound, and in that case it may be preferable to utilize only the two front speakers. It is still desirable to use all four amplifiers.
Heretofore, switching systems have been provided between the four amplifiers of a 4-channel playback system and the respective loudspeakers to disconnect the back speakers and to apply the output signals from the corresponding amplifiers to the front speakers. Thus, the amplifier that would normally feed the right-back speaker is connected to the right-front speaker and the output signal from the amplifier that would normally feed the left-back speaker is connected to the left-front speaker. The two back speakers have simply been left in an open-circuit condition. Moreover, the amplifiers, being of balanced transformerless type, have been connected so that, in the 2-channel mode, the outputs of the left-back amplifier and the left-front amplifier connected to the left-front speaker are connected differentially to that speaker and the right-back and right-front amplifiers connected to the right-front speaker are also differentially connected to that speaker.
Each speaker has two input terminals and each amplifier has two output terminals, one of which is a common terminal such as ground. In the differential connection, the ungrounded, or "hot" signal output terminal of the front and back balanced transformerless amplifiers are connected to the two terminals of the corresponding front speaker. As a result, this speaker is energized with signals of doubled magnitudes so that the reproduction of sound from that speaker will be at a higher level than in the operating mode in which each speaker receives its energizing signal from only one amplifier.
The switching system used heretofore have had the disadvantage of complexity of two double-throw switches connected together and the circuit connections, leaving the supposedly unenergized back speakers in an open circuit condition, have made it possible for those speakers to pick up signals by stray capacitance and thus produce an undesired sound.